Phase modulation of an optical beam finds important usage in interferometry, fiber-optic gyros and sensors, switchable lenses, reconfigurable highly parallel optical interconnects, phase and phase gradient array beam steering (LADAR), as well as in other optical devices.
Presently, phase modulation is accomplished either by winding hundreds of meters of optical fiber around a piezoelectric cylinder or by electro-optically changing the index of refraction in electro-optic materials such as Nematic Liquid Crystals or Lithium Niobate. Both present approaches to phase modulation have disadvantages. Use of the piezoelectric cylinder is bulky due to the amount of optical fiber required for phase modulation and is not very sensitive. The electro-optical change in the index of refraction of optical materials is plagued by high losses, low throughput power, single polarization, temperature sensitivity and high costs.
Yet, a main advantage of optics and optical devices is their electromagnetic passivity, that is, the nonreactivity of waves of photons with each other or with electromagnetic waves. Thus, to find a new, more sensitive, and less bulky method of accomplishing phase modulation in optical materials is desirable.
Unfortunately, the electromagnetic passivity of optics works to a disadvantage when one attempts to alter the position or phase of an optical wave. Attempts at optical wave modulation include a method of electrically changing the index of refraction of the material through which the beam is propagated. Results to date have produced a small amount of optical wave modulation which operates on only a single mode and polarization and still incurs high losses.
In U.S. Pat. No. 5,016,957, columns 2, 3 and 4, lines 0-68, and column 5, lines 0-60 inclusive, issued May 21, 1991, and U.S. Pat. No. 5,095,515, columns 2, 3, and 4, lines 0-68, and column 5, lines 0-60 inclusive, issued Mar. 10, 1992, hereby incorporated by reference, stress-optical switches made from photoelastic, transparent optical materials were described. The optical material was subjected to a force that formed a mechanical stress gradient and a resultant non-uniform index of refraction gradient within the material. The index of refraction gradient so formed altered the optical path of the optical beam passing through the optical material to form an optical switch. However, stress-optical switches did not achieve phase modulation.